[DocDB] Reduce boilerplate for defining hash functions for structs

[es1024]

Jira Link: DB-14634

Description

Defining hash functions for structs requires a lot of boilerplate. For example, to make
boost::hash work with YsqlFullTableName, we have:

inline size_t hash_value(const YsqlFullTableName& table) {
  size_t value = 0;
  boost::hash_combine(value, table.namespace_name);
  boost::hash_combine(value, table.schema_name);
  boost::hash_combine(value, table.table_name);
  return value;
}

This can be reduced with macros similar to YB_STRUCT_TO_STRING.

Issue Type

kind/enhancement

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